The present invention relates to the field of packaging semiconductor devices, and more particularly to packaging light emitting diodes.
Light emitting diodes (LEDs) are often packaged within leadframe packages. A leadframe package typically includes a molded plastic body which encapsulates an LED, a lens portion, and thin metal leads connected to the LED and extending outside the plastic body. The metal leads of the leadframe package serve as the conduit to supply the LED with electrical power and, at the same time, may act to draw heat away from the LED. Heat is generated by the LED when power is applied to the LED to produce light. A portion of the leads extends out from the package body for connection to circuits external to the leadframe package.
Some of the heat generated by the LED is dissipated by the plastic package body; however, most of the heat is drawn away from the LED via the metal components of the package. The metal leads are typically very thin and have a small cross section. For this reason, capacity of the metal leads to remove heat from the LED is limited. This limits the amount of power that can be sent to the LED thereby limiting the amount of light that can be generated by the LED.
To increase the capacity of an LED package to dissipate heat, in one LED package design, a heat sink slug is placed under the metal leads within the LED package. The heat sink slug increases the capacity of the LED package to dissipate heat; however, the heat sink slug increases the size, the mass, and the cost of the LED package. Increases in the size, the mass, and the cost are undesirable.
In another LED package design, the leads of the leadframe are extended (in various shapes and configurations) beyond the immediate edge of the LED package body. This increases the surface area of the portions of the leads exposed to the surrounding air. The increased exposed surface area of the extended leads increases the capacity of the LED package to dissipate heat; however, the extended leads increase the size, the mass, and the cost of the LED package.
Another undesirable aspect of the current leadframe package design relates to problems associated with thermal expansion of the package. When heat is generated, the LED package experiences thermal expansion. Each of the parts of the LED package has a different coefficient of thermal expansion (CTE). For example, the CTE of the LED, the CTE of the package body, the CTE of the leads, and the CTE of lens are different from each other. For this reason, when heated, each of these parts experience different degrees of thermal expansion resulting in mechanical stresses between the parts of the package thereby adversely affecting its reliability.
Consequently, there remains a need for an improved LED package that overcomes or alleviates one or more of the shortcomings of the prior art packages.